The present invention relates to optical communication systems employing wavelength division multiplexing (WDM), and more particularly to systems and methods for conditioning optical signals at a WDM receiver.
WDM techniques are finding increasing application in optical networks. When WDM is used, multiple optical signals at different wavelengths are combined on a single fiber. This type of operation greatly increases the data carrying capacity of a single fiber. It is also then possible to add and drop individual wavelengths using optical techniques and without having to convert all the optical signals to electrical form.
In a WDM system, when it necessary to convert the multiple optical signals to electrical form, either for regeneration or for data recovery, they must be demultiplexed onto separate fibers. Once separated, the individual optical signals are converted to electrical form by photodetectors. Separating the individual signals from one another is the job of an optical component known as a demultiplexer. For optimal detection, the optical signals should have power levels within the dynamic range of the photodetectors. In some representative systems, the receiver dynamic range may be on the order of 8 dB. Since the optical signals have typically traveled a large distance without regeneration before encountering the demultiplexer and photodetectors, amplification will be necessary to achieve the necessary power levels.
The level of amplification will depend on the optical channel from the transmitter to the receiver and will vary over time. If too little amplification is used, the signals will be swamped by noise leading to bit errors in data recovery. If too much amplification is used, the detectors (or the electronic components following the detectors) will saturate, again corrupting data.
One solution to the dynamic range problem is to amplify each optical signal individually following separation by the demultiplexer. Amplifier gain is then controlled for each wavelength so as to assure that each wavelength signal is kept within the dynamic range of its photodetector. This would require that amplifier and AGC components be provided for each wavelength used. With the advent of dense WDM (DWDM) techniques where numerous closely spaced wavelengths are used, this type of solution becomes unworkable.
To optimize use of amplifier components it is preferable to place the necessary pre-amplification prior to the demultiplexer so that only one amplifier operates on all of the wavelengths. This, however, complicates gain control because amplification characteristics will not be flat over wavelength and it will be difficult to set the amplifier gain so that each WDM channel is within the dynamic range of its photodetector. One approach is to simply set gain based on a measurement of the total amplifier output power. Because the amplifier gains vary over frequency, however, this approach may cause some channels to be either above or below the receiver dynamic range. Other approaches rely on the use of filters with variable response characteristics to reduce such amplifier gain variation.
What is needed are systems and methods for controlling the power level of individual WDM channels at the receiver at low cost. The power control system should also readily adapt to large numbers of WDM channels.